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PostPosted: Thu Sep 12, 2019 10:36 am 
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I wanted to reach out on a few places, so I'm going to paste some of what I had originally posted on the vintage mustang forums here.

Ok, so I'm not 100% sure we have a vacuum leak now.

Our 351c is freshly rebuilt, by high schoolers mind you.



We have a Pertronix Flamethrower Ignitor 1 distributor(D134620) with matching coil.

Crane cam H-278-2 with matching lifters.

Stock pushrods, rockers, etc. Stock springs, all set to correct height by our machine shop.

Currently running the stock intake.

Right now she is hard to start, we get some backfires out of both ends. I retarded the timing some and it seemed to help. We set base timing at 10 degrees BTDC. We droped a new carb on it it it is getting good fuel now. She doesn't want to idle very well below 800 rpm.

Once it is a little biy more stable I plan to try a power balance test to see if all the cylinders are firing equally.

We have not done a compression test as of yet.

We did set the adjustable timing gear to 2 degrees BTDC as we were suggested after looking at our cam specs.


On thing I am concerned about is the valvetrain geometry and pushrod lengths. I don't think they should be a problem, but maybe? If valves weren't seating, that would explain backfires. We have non adjustable, stock, rockers.

We can do a compression test today...

EDIT- we did a quick run this morning and found we had 15"hg at 1200rpm, and then it bounced between 5-10 at 800rpm. You can observe it in the video-
https://youtu.be/1YOW2qYfs6U


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PostPosted: Fri Sep 13, 2019 7:46 am 
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Check the plug wires for correct location and crossfiring. Looks and sounds like you have a misfire somewhere causing the rough running. That would also explain the hard to start, and backfires out of both ends, the bouncing vacuum and how it seems to smooth out at 1200.

After you go through all of that, use an infrared temp gun on the exhaust to find which cylinders aren't firing.


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PostPosted: Fri Sep 13, 2019 9:20 am 
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In the video tt looks like the wires are on right to me 1,3,7,2,6,5,4,8

I think it has an intake valve one or several that are too tight and not seating. You should be able to find this in a leak down test. If that's the case just make a spacer about .030-.060 or so thick and put it under the rocker pedestal on the offending valve to jack it up a little bit.


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PostPosted: Fri Sep 13, 2019 2:46 pm 
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Location: Texas
Try unhooking the vacuum advance and see what it does then (of coarse plug the vacuum port on the carb). Vacuum advance is not good with every cam. I had an old Wolverine 290/290 .505/.505 on a 110 lobe. I ran the Duraspark distributor. The engine would run like crap with the vacuum advance. I took it off and put the initial timing at 16 degrees and it ran hard as hell.

Maybe im wrong but Im not sure about how or why you set this aftermarket cam to factory cam like 10* BTDC specs. Heck I think the factory 2v Cleveland was 6* BTDC. Did you align the timing gears so they meet even in the middle?


Last edited by nova467spanker on Fri Sep 13, 2019 6:14 pm, edited 1 time in total.

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PostPosted: Fri Sep 13, 2019 5:40 pm 
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What heads? If those are open chamber heads and the compression isn;t very high, you need to be starting in the 16~18 initial range, not 10. For an OC engine, 10 is nearly running backwards LOL.

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PostPosted: Fri Sep 13, 2019 8:46 pm 
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Make sure 5&6 spark plug wires are separated good they are consecutive in the firing order. What does the inertia ring look like on the balancer is it cracked? I would not trust a stock balancer (if it is stock) to read correctly, add some timing or take more away and see if it gets happier it might be off a tooth on the distributor. Verify TDC with a plug style piston stop clockwise and counterclockwise divide the degrees by 2 and mark your 0 degrees (timing tape if needed). Lights out running is a good test for crossfire, hit the manifolds with a temp gauge look for a cold cylinder(s). Make sure the transfer slot is not open too far, look at the rear throttle blades and see if the transfer slot looks like a square and the blades are mostly closed.When you add timing the idle should pickup and you should have to adjust the idle down and readjust the mixture 1 1/2 turns out is a good starting point.

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PostPosted: Wed Sep 18, 2019 7:10 am 
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DaveMcLain wrote:
In the video tt looks like the wires are on right to me 1,3,7,2,6,5,4,8

I think it has an intake valve one or several that are too tight and not seating. You should be able to find this in a leak down test. If that's the case just make a spacer about .030-.060 or so thick and put it under the rocker pedestal on the offending valve to jack it up a little bit.


After the test and, if needed, after the shim idea per Dave, if it still backfires I'd pull the timing chain cover and go over the cam timing again.


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PostPosted: Wed Sep 18, 2019 9:27 am 
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I think if the valves were hanging over, it would struggle to even start in the first place because of a lack of compression.

My money is on a spun balancer and the timing marks being off, causing the ignition timing to be retarded.

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PostPosted: Thu Sep 19, 2019 9:38 pm 
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I’d try advancing the timing. Went through the same thing recently. Thought I had the timing set right and the old balancer had slipped. Got a good balancer on it and it was trying to fire atdc causing the backfires. Couldn’t believe it would even run.


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PostPosted: Fri Sep 20, 2019 5:11 pm 
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Zrob90sc wrote:
I’d try advancing the timing. Went through the same thing recently. Thought I had the timing set right and the old balancer had slipped. Got a good balancer on it and it was trying to fire atdc causing the backfires. Couldn’t believe it would even run.
\

Good point by you and Brent. If OEM, heck those balancers are almost 50 years old and that rubber-like material between the hub and out ring could be shot.


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